Currently, Liquid Crystal Displays (abbreviated as LCD) are usually used in terminals, and the backlight brightness of the LCDs greatly affects the imaging effects. Light Emitting Diodes (LED) are applied as backlight emitting devices, and usually, an anode of a LED is connected to a positive pole of a battery, and a cathode of the LED is in series connection with a brightness balancing resistor and then connected to a current control end.
The number of the LEDs required by the terminals is relevant to the display area of the LCD and the emitting power of the LED. However, there is a disadvantage in this circuit: the battery voltage gradually reduces over the working time, and when the battery voltage reduces to a certain extent, the backlight brightness cannot be ensured; in addition, when the power of the whole machine fluctuates, the backlight brightness could also fluctuate. For example, when an entertainment terminal plays a video and uses a built-in loudspeaker, if the battery voltage is insufficient, the backlight brightness could flicker with the music, and the lower the battery voltage, the higher the brightness level, the greater the volume of the loudspeaker, the more apparent the fluctuation of the backlight brightness.
In order to solve the above problem, in the related art, a backlight circuit with a boost circuit is employed in some terminal products, and the advantage of such circuit is: when the battery voltage is relatively low, the LED backlight can still obtain relatively high working voltage from the boost circuit. In this way, the backlight brightness could not flicker with the fluctuation of the power of the whole machine.
Although, the backlight circuit with a boost circuit solves the problem that the brightness fluctuates when the power of the whole machine fluctuates under a low voltage condition, the additional power consumption is often brought along. For example, for a terminal product, one lithium battery is employed to supply power, the voltage of the lithium battery is about 4.2 V when being fully charged, and the voltage for the terminal to power off automatically is about 3.3 V. The boost circuit of the backlight circuit of the terminal provides a fixed 4.3 V voltage to the LED. The brightness of some terminals can be regulated, it need not provide a 4.3 V voltage to the backlight under low brightness condition, the excessive voltage can be consumed at the control end, which results into heat loss and reduces the endurance ability of the terminal.